This invention relates to fine particles of hydrophilic diamond which can effectively provide a single layer of particles when used in lubrication, surface mondification, abrasive applications, etc. The invention also relates to a method of effectively producing such diamond.
Diamond fine powders less than one micrometer in nominal average particle size (APS) are available on the market as a grinding medium, and even superfine powders of 5 nm (nanometers) for lubrication, surface modification and polishing applications. They are often used with a specific dispersion medium, in order to achieve a uniform deposit of single particle thickness on a support body, or a regular distribution of separate particles in the matrix.
Diamond particles are often at a decreased surface energy level due to the combination with different atoms or groups, although the surfaces may be sometimes made up of carbon atoms joined with each other. Thus the physical and chemical properties of diamond particles are affected by the surface state, increasingly with the decrease in particle size, and essentially for submicron sizes, in addition to their bulk properties. So the dry agglomeration and affinity to aqueous or oily medium of particles, in particular, depend essentially on the atoms or groups, adsorbed or combined with carbon atoms in the surface layer. Possible atoms and groups are collected and out-lined, along with techniques for providing them, by N. V. Novikov in xe2x80x9cThe Physical properties of diamondxe2x80x9d, Naukova Dumka (1987).
Diamond fine powders are commonly prepared by crushing coarser particles of either natural or synthetic origins, the latter being produced by static compression at high temperatures on a hydraulic press or dynamic compression by means of explosive detonation. They, natural or synthetic, assume a set of specific properties which vary with the different set of physical and chemical processes they were subjected on the way to the end products.
Diamond from a hydrostatic press, for example, contains metallic or nonmetallic substances which derive from the flux and specimen accommodation materials employed in the process, and which exist in the crystals as an impurity. Some kinds of impurity may be removed and the powder be purified to a degree, as the particles undergo chipping or splitting during the crushing process, preferentially at crystal defects and expose the foreign substance within, which is to be dissolved during the acid treatment. However there are other impurities, including chromium from the crushing machine and graphite remaining unconsumed and trapped in the particles which then are exposed or released by the breakdown. As insoluble to the acids, they accumulate in the solution and accompany with the finest fraction when recovered. As a result, the class zero diamond powders, which contain the undersize end, often exhibit a light to dark grayish color due to those impurities.
As for the dynamic compression synthesis technique, whose mechanism has not fully been understood, diamond fine powders contain impurities whose origin is not always identified. However there are some elements, beside graphite, which should evidently derive from the specimen receptacle material or, at least, matrix material for the process.
Synthetic diamonds in general may be somehow more hydrophilic than natural ones as a result of the treatments the former undergo, before it is isolated as separate particles, with or in various chemical solutions which can leave oxygen atoms or hydroxy groups, combined with carbon atoms on the particle surface. Although such quality even causes a trouble in establishing a reproducible size grading of micron sizes by affecting Stokes""s relationship between particle size and sedimentation rate in the elutriation process. If so, the treated diamond particles in general are not in a surface state which would form a stable suspension or dispersion in a polar solvent such as water and alcohol.
Besides oxygen and hydroxy atom and group, presence of several elements as impurities has been recognized by ICP spectrometry or inorganic qualitative analysis. They include Si, Al, Fe, Cr, Mn, Cu, Ca, S and C. It appears that some of them are somehow interactive with the surface of diamond particles whichever the production process is, and causes agglomeration of said particles in the aqueous environment. Thus impurities make most fine diamond powders so far available tend to form an agglomeration, which is almost impossible to completely break down in polar or nonpolar medium, even under ultrasonic vibration. It is further observed that such fine particles, once separated, readily gather and form again an agglomeration and begin to sediment.
Diamond fine powders are promising materials for the use in tribologic and abrasive applications as well as surface modification, if they can be successfully separated into single particles and distributed in a matrix or spread on a carrier body. While it is understood that such quality can be achieved by either eliminating the above said interaction or by improving the wettability of the diamond surface with the medium employed, no effective techniques have been available for the purposes.
Metal or ceramic materials coated with diamond fine particles could be useful as a wear resistant material. It is essential, however, to keep the diamond particles in stable suspension, with the diamond concentration or particle distribution unvarying over the deposition process, for example.
Therefore one of the principal objects of the invention is to provide diamond fine particles with a surface nature so improved as to form a stable, uniform suspension or dispersion in a common medium such as water and alcohol.
Another object is to provide an effective technique for producing hydrophilic diamond fine particles by chemically modifying the particle surface nature, while removing at the same time contaminants and foreign materials which coexist with the diamond.
In the invention diamond particles are treated by boiling in the treatment fluid of sulfuric acid solution, which is in particular of concentrated or fuming nature, at a temperature more than 200xc2x0 C., which is preferably 250xc2x0 C. or more. Thus hydrophilic diamond fine particles are left and recovered with hydrophilic atoms and/or groups formed on the surface, while contaminants and foreign matters are removed from among the particles or from the surface as chemically decomposed and dissolved into the solution.
Said treatment fluid may further comprise one or more of nitric, perchloric and permanganic and other inorganic acid as an oxidizer, as well as potassium or other metal nitrate. They are used essentially in combination with concentrated or fuming sulfuric acid, in order to achieve a corresponding effect at lower temperatures.
Results are more appreciable and the increase in suspension stability is greater with finer particles. Coarser particles are more susceptible to the gravity and thus are more difficult to hold a good suspension in a medium for a sufficient time. Thus the method of the invention employs average particle sizes ofxe2x80x94or less than 2 xcexcm (micrometer), and the diamond particles as treated hold suspension practically without sedimentation in purified water for, at: least, 30 minutes with the 2 xcexcm size and more than 2 hours with 1 xcexcm or less size. Further finer particles can exhibit a further longer suspension holding time of, for example, more than 24 hours for a size of 200 nanometers or less. A suspension time of 6 hours or more also is achieved with such ultrafine particles in a pH 4.0 acidic aqueous solution of inorganic acid. Hydrophilic diamond surface which allows such good suspension is produced by the method of the invention at a good reproducibility.
Since polar media such as water and alcohol are commonly used for distributing diamond particles in, the latter should be more favored as versatile if, and when, they are of hydrophilic nature and, at the same time, free of contaminants or foreign matters on or among the particles. If the impure diamond particles tend to agglomerate in acidic media, the alkaline solution connot always be applicable but just sometimes employed, for the purpose of keeping the particles in suspension.
The distribution of diamond particles in an aqueous medium is also affected by the presence of some ion species. Thus hydrophilicity is essentially evaluated in the invention in terms of suspension stability or the duration for which the suspension is sustained in a purified aqueous medium at pH=7.0 of deionized or distilled water.
The diamond particles of the invention is produced, essentially, by providing fine diamond particles of APS of or less than 2 xcexcm, placing and heating said diamond particles in an oxidizing medium at a temperature of at least 200xc2x0 and, preferably, 250xc2x0 C., whereby hydrophilic atoms and/or functional groups such as Cxe2x95x90O and OH, for example, are provided on the furface of the diamond. Contaminants and foreign matters are also removed at the same time.
The treatment temperature should not exceed 350xc2x0 C. since oxidization loss is noticeable at said temperature and more, with particle size less than 1 xcexcm.
For coarser particles greater than 1 xcexcm, a higher temperature of 400xc2x0 C. is considered as the upper limit because of the equipment material availability and process controllability, although the oxidization loss becomes significant only above 500xc2x0 C., approximately.
In the sulfuric acid treatment of the invention metallic impurities and graphite are oxidized with SO3 to be dissolved as a sulfate and to form a gaseous product. A high acidic aqueous solution, which forms when the treatment solution is diluted with water upon the termination of the process, is very effective for the removal of the metallic impurity and thus the probable nuclei and origins of particle agglomeration. At the same time oxygen atoms are combined with carbon atoms to form Cxe2x95x90O bonds, or further OH groups by the subsequent hydration on the treated diamond particles, which as a result should probably acquire a hydrophilic nature. Such effects are more apparent with finer particles, with a stable suspension in a pH 4.0 acidic solution of inorganic acid holding at least 6 hours with an average particle size of or less than 200 nm.
Nitric, perchloric, chromic and permanganic acid as well as potassium or other metal nitrate are available singly or in combination as an oxidizer and added to the basic solution of sulfuric acid, in order to decrease the treatment temperature. With the oxidizer, as a surface analysis showed the presence of Cxe2x95x90O and OH, it can be assumed that a corresponding process takes place as in the oxidizer-free fluid but at lower temperatures.
It is not essential to the invention but optional as desired, treatment with perchloric, hydrofluoric acid or in molten alkaline salt may be added either before or after the sulfuric acid treatment, in order to remove principally graphite, silica or silica and alumina, respectively.
The hydrophilic fine powder of diamond of the invention exhibits good resistance to agglomeration: it disperses, apparently in single particles as soon as put in water, to form an opaque fluid. In fact a particle size measurement suggests that they are distributed completely in single or little agglomerated particles, and thus they are covered with hydrophilic atoms or groups. Breaking down of the powder by ultrasonic vibration or the like is usually unnecessary.
Since the diamond particles of the invention, as described above, exist in the suspension practically in single or little agglomerated particles, they can be used to form a monoparticle layer or a mixture of high uniformity with another particle substance. Here the concentration can be controlled, as necessary, or a homogeneous mixture with a monomer in the production of plastic material can be achieved by using a solvent which exhibits mutual solubility to water.
The diamond powder of the invention can be used in various ways. A starting material of conventional clustered diamond powder can be treated by the method discribed above to prepare a suspension in water/alcohol mixture. A sheet of silicon is dipped to deposit a layer of diamond fine particles, and used in a diamond CVD process as a substrate with a high concentration of nucleation sites.
By a similar technique a sheet of metal or plastic, for example, may be dipped in or spread over with suspension of submicron diamond particles, in order to prepare an abrasive sheet practically free of irregularity in particle size or distribution.
An abrasive tool with a few micrometer diamond particles uniformly distributed in the matrix can be produced by first preparing a suspension of treated diamond particles, which then is mixed with a fluid or powder of matrix material.
Now the invention is illustrated by means of examples, in which the starting point of sedimentation is defined as the time when a clear top has become noticeable in the observation in a test tube, left still, of 25 ml sample suspension, which is taken from the bulk prepared from 200 mg of diamond particles in 100 ml of purified water of pH=7.0. Particle sizes are evaluated by centrifugal sedimentation for the 300 nm or less, while laser diffraction scattering technique was used for the coarser particles.